ProjectSLL

SLLed

A decorative lighting piece I designed as a gift for an old ex. That relationship has ended, so I thought I could make it a part of my public portfolio and put it to use in finding a job (help).

The project is called "SLLed" because the intended recipient had supposedly caught me talking about singly-linked lists in my sleep. wtaf.

Inspired by https://www.instructables.com/DIY-LED-heart/.

Hardware

I first started by building a schematic in KiCAD. Some design considerations:

• I was at uni and didn't have access to my usual soldering equipment, so I had to get my PCB assembled by its manufacturer. This meant that my component selection was restricted to what the manufacturer had available.

• I also didn't have access to any mcu programming tools, so I wanted this module to be USB-programmable for ease of development. USB-C because, c'mon guys, it's 2023.

For the MCU I settled for the ATMega16U2, which is from the family of microcontrollers used in the classic Arduino line (the commonly-used 32U4 was out of stock).

I added a 16MHZ crystal should I choose to implement some form of USB connectivity with a host device in the future (for which the 16U2's internal crystal would be insufficient).

I also designed a very simple MOSFET-based power-switching circuit so that this module can be powered both via battery and USB.

The final schematic may be found here.

I then assigned footprints to components and build a quick layout. Some design considerations:

• For reasons of aesthetic purity, everything visible on the front-side of the PCB must be part of the heart. No vias outside of the heart!

• Added a small hole to the top of the PCB, so you could thread a string through it and hang the module on somewhere

The geometry of the heart took some time to get right, but it turned out ok.

I exported the Gerber files and BOM, and sent it off to JLCPCB for manufacture. Here are some renders:

Software

In my humble opinion, using Arduino as a university student in computer science is akin to intellectual cowardice. It is a sign that one lacks to bravery to face the almighty datasheet. /s

I programmed this in AVR C because I wanted to learn more about how microcontrollers worked at a lower level. Implementing I/O functions and interrupts by directly interacting with registers was a fun challenge and made the end result all the more rewarding.

The code is structured as such:

• I have system of decoding arrays so that the corresponding bits at the corresponding registers for each LED can be easily accessed via an index, where 0 is the top LED and the index moves through the heart clockwise.

• The program runs on an infinite loop, which relies on a frame counter that increments and resets periodically. For each lighting mode, I have done some rudimentary maths to map the frame counter to the indices of the LEDs that need to be turned on or off.

• An interrupt service routine is defined such that whenever the push-button (connected to Pin C2) is pressed, the loop resets and and the lighting mode changes.

There are currently 5 lighting modes built into the program : twinkle, heartbeat, vertical sweep, snake, and solid (hopefully these names are all self-explanatory).

Gallery